There are various types of network configurations, including cellular networks, ad-hoc networks, or a combination of both. In the case of the cellular network, the user equipment communicates (e.g., transmits and/or receives) with another user equipment through a base station. In the case of the ad-hoc network, the user equipment communicates directly with another user equipment. Ad hoc networks are also called device-to-device (D2D) networks, which refers to the direct link(s) between user equipment.
Some D2D communications are also controlled by a base station, providing so-called “cellular controlled” D2D communications (which is also referred to as cellular assisted D2D communications). In cellular controlled D2D communications, two devices may be directly linked via a D2D connection, and the devices may be attached to a base station, such as an enhanced Node B (eNB) base station, to exchange control information with the eNB (or other nodes of the network). The control information may include, for example, mode selection information, such as whether D2D communications or cellular connections should be used, resource allocation for the D2D connections, handover information, power control related information, commands (e.g., from the eNB to the D2D devices), resource allocation requests, and/or measurement reports from the D2D devices to the eNB base station.
When two devices are in D2D communications, one of the devices may be handed over to another cell. For example, the first device of the D2D pair may be connected to a first eNB base station, while the second device of the D2D pair may be connected to a second eNB base station. When this is the case, the D2D communications is referred to as a multi-cell D2D connection. The multi-cell D2D connection requires resource allocation not only between the two D2D devices but also between the adjacent cells (e.g., the first and second eNBs). Moreover, the D2D devices are often in close proximity to each other, so it might be useful for the network (e.g., the eNB or other network nodes) and the D2D devices to consider a joint handover to another cell to avoid the multi-cell D2D connection. Although multi-cell D2D connections can be used, initiating separate handovers for each device may cause extra burden for the device and the network in terms of additional control signaling and decreased efficiency of the D2D connection.